The present invention relates to an earth-working machine such as an excavator for digging ditches in road construction, the earth-working machine having a turntable and a carriage which are controllably angularly movable independently on a self-propelled mobile chassis for enabling an excavating mechanism on the carriage to turn in various angular ranges for avoiding interference with surrounding traffic and/or objects and providing wide working areas for the excavating mechanism.
Conventional earth-working machines or excavators include an excavating mechanism composed of a boom or bucket arm having a bucket on its distal end for trenching a ditch in a road. In operation, the material scooped by the bucket is transferred back by turning the boom around the machine. Since the boom is angularly moved through an semicircular angular range, the boom and the bucket as they move project laterally of the machine, resulting in the danger of interfering with surrounding traffic and/or objects. Therefore, a large working radius or range clear of any obstructions should be reserved around the machine for allowing safe swinging movement of the boom. This requirement however is difficult or eve impossible to meet in situations where only relatively small or limited spaces are available for the machine.
To cope with such a difficulty, there has been proposed an excavator having a turntable rotatably mounted on a mobile chassis and a carriage rotatably mounted on the turntable and supporting an excavating mechanism, the turntable and the carriage having shafts positioned out of coaxial relation. With this arrangement, the bucket on the boom is allowed to move over the chassis without projecting laterally thereof when the turntable and the carriage are turned about their shafts. Therefore, unwanted interference with traffic or objects is prevented around the machine, and the excavator can be placed in relatively small spaces for road construction or other earth-moving applications. The proposed excavator however still suffers the drawbacks in that the turntable and the carriage have to be rotated in predetermined directions in synchronism with each other in order to cause the bucket to pass over the chassis, and a relatively complex mechanism is necessary for turning the turntable and the carriage in such a manner. The conventional turning mechanism comprises a mechanical driving assembly composed of gears for actuating the turntable and the carriage synchronously with each other. However, such mechanism allows stresses to be localized in certain parts, which tend to be damaged or otherwise fail, and also undergoes increased friction resulting in a large power loss and poor efficiency.